A gas or combustion turbine is a type of internal combustion engine. Air is compressed within a compressor and fuel is injected into the air stream in a combustor or combustion chamber where ignition occurs. Ignition of the fuel creates a high-pressure and high-velocity gas flow that is directed to a turbine, causing it to rotate.
Air is supplied to a diffuser having a larger cross-sectional area at a diffuser exit than at a diffuser entrance. Air exiting the diffuser has both a lower velocity and lower dynamic pressure than incoming air. The diffuser opens directly into the compressor where the air is compressed and accelerated.
The compressed air is directed to the combustion chamber comprising a ring of fuel injectors that direct fuel into the compressed air stream to ignite the air/fuel mixture. Ignition increases both the temperature and pressure of the air/fuel mixture (also referred to as a working gas). Most gas turbines are designed to combust a broad range of hydrocarbon fuels, such as natural gas, kerosene, biomass gas, liquid natural gas, synthetic gas, etc.
The working gas expands as it enters the turbine, which includes rows of stationary guide vanes and rotating blades connected to a turbine shaft. The expanding gas flow is accelerated by the guide vanes and directed over the rotating blades, causing the blades and the turbine shaft to spin. The spinning shaft both turns the compressor and provides a mechanical torque output. Energy can be extracted from the turbine in the form of shaft power, compressed air, thrust or any combination of these for use in powering aircraft, trains, ships and electric generators.
After passing through the turbine, the working gas flow enters a turbine exhaust casing comprising an exhaust gas diffuser and a nozzle through which the exhaust gases flow.
Referring to prior art FIG. 1, a gas turbine 10 generally includes a compressor 12, a combustion chamber 14, a turbine 16 and an exhaust casing 18. The compressor 12 inducts and compresses ambient air. The compressed air then enters one or more combustors 20 in the combustion chamber 14, where the compressed air is mixed with fuel. The air-fuel mixture ignites to form a hot working gas. The working gas is directed to the turbine 16 where it expands through alternating rows of stationary guide vanes and rotating blades to generate mechanical forces that turn a shaft 26. The expanded gas exiting the turbine 16 is exhausted from the engine 10 via the exhaust casing 18.
According to one implementation, the exhaust casing 18 comprises an exhaust diffuser 28, i.e., a divergent duct formed by an outer shell 30 and a center body or hub 32, and further comprises a tail cone 34. Support struts 36 span the space between and are affixed to the outer shell 30 and the hub 32. The exhaust diffuser 28 is shaped to reduce the speed of the exhaust flow and to increase the pressure of the exhaust gas exiting the exhaust casing 18. In some prior art turbine exhaust gas casings, this exhaust diffusion is achieved by progressively increasing the cross-sectional area of the exhaust duct in the direction of fluid-flow, thereby expanding the fluid during transit though the exhaust gas casing 18.
In one embodiment the outer shell 30 is about 13 feet in diameter and about 14 feet long. The center hub is about 6 feet in diameter at the largest diameter and about 14 feet long.
Typically, upstream sections of the outer shell 30 and the center hub 32 are each fabricated in two half sections, i.e., an upper section and a lower section. The upper section comprises both the outer shell upper section and the center hub upper half section, as these two sections are joined by the support struts 36 that extend between the outer shell upper section and the center hub upper section. The lower half section similarly comprises both the outer shell lower half section and the center hub lower half section with struts 36 connecting the two lower half sections.
As illustrated in FIG. 2, an upper half section 30A of the outer shell 30 comprises a first and a second row of flanges 50 (also referred to as scalloped flanges due to their shape) extending outwardly along respective edges 30B and 30C of the upper half section 30A.
On an upper half section 32A of the center hub 32, a third and a fourth row of flanges 50 extend inwardly along respective edges 32B and 32C.
The lower half sections of the outer shell 30 and the center hub 32 (not illustrated) are similarly configured with flanges for contacting the flanges 50 of the upper half sections 30A and 32A. A coupling element, such as a bolt/nut combination, is inserted into through-holes or openings in each pair of contacting flanges 50 for coupling the upper and lower half sections of the outer shell 30 and the center hub 32.